Advanced Programming in the UNIX® Environment

ptg10805159

556 Interprocess Communication Chapter 15

client ... client

well-known

FIFO

server

readrequests

client-specific

FIFO

client-specific

FIFO

write

replies

write

replies

readreplies readreplies

write

requests

write

requests

Figure 15.23Client–server communication using FIFOs

server also must catchSIGPIPE,since it’s possible for a client to send a request and

terminate beforereading the response, leaving the client-specific FIFO with one writer

(the server) and no reader.

With the arrangement shown in Figure15.23, if the server opens its well-known

FIFO read-only (since it onlyreadsfromit) each time the number of clients goes from 1

to 0, the server willreadan end of file on the FIFO.To prevent the server from having

to handle this case, a common trick is just to have the serveropenits well-known FIFO

for read–write. (See Exercise 15.10.)

15.6 XSI IPC

The three types of IPC that we call XSI IPC—message queues, semaphores, and shared

memory — have many similarities. In this section, we cover these similar features; in the

following sections, we look at the specific functions for each of the three IPC types.

The XSI IPC functions arebased closely on the System V IPC functions. These three types of

IPC originated in the 1970s in an internal AT&T version of the UNIX System called ‘‘Columbus

UNIX.’’These IPC features werelater added to System V.They areoften criticized for

inventing their own namespace instead of using the file system.

15.6.1 Identifiersand Keys

Each IPC structure(message queue, semaphore, or shared memory segment) in the

kernel is referred to by a non-negative integeridentifier.Tosend a message to or fetch a

message from a message queue, for example, all we need know is the identifier for the

queue. Unlike file descriptors, IPC identifiers arenot small integers. Indeed, when a